<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>dispaly-flex</title>
    <style>
        div.parent {
            display: flex;
            justify-content: center;
            align-items: center;
        }
        /* div.parent {
            font-size: 0;
            text-align: center;
            
        }
        div.parent::before {
            content: "";
            display: inline-block;
            width: 0;
            height: 100%;
            vertical-align: middle;
        }
        div.child{
            display: inline-block;
            vertical-align: middle;
            background-color: aqua;
        } */
    </style>

</head>

<body>
    <div>
        <div class="parent">
            <div class="child">
                1346455
            </div>
        </div>
    </div>
</body>